267
Homogeneous Photo-Fenton Reaction
The Fenton reaction is a process that does not involve any light irradiation as compared with the heterogeneous TiO 2 photocatalysis reaction, whereas the photoFenton does react up to a light wavelength of 600 nm. It was first recognized in the
1960s and remains one of the most applied AOPs for its ability to degrade high
loading of organic compounds in highly saline conditions [17, 202, 243]. Numerous
studies on the photo-Fenton degradation of water pollutants such as chlorophenol,
pesticides, and phenolic or aromatic compounds with organic loading of up to
25 g/L have been investigated [88, 107, 108, 134, 259]. A number of literatures have
provided a comprehensive review of the basic understanding and clarity of the principles underlying the Fenton reaction [111, 243, 262].
In the absence of a light source, hydrogen peroxide (H 2 O 2 ) will be decomposed
by Fe
2+
ions that are present in the aqueous phase, resulting in the formation of
hydroxyl radicals. The photo-Fenton reaction is expedited when light source is present, causing rapid H 2 O 2 decomposition by ferrous or ferric ions and resulting in the
formation of radicals. All these soluble iron hydroxy or iron complexes can absorb
not only UV radiation but also visible light. However, the actual oxidizing species
responsible for the photo-Fenton reaction is still under discussion [42, 73]. These
Fenton and photo-Fenton reactions could occur simultaneously with TiO 2 photocatalysis during UV-Vis irradiation period, post-TiO 2 photocatalysis period, or standalone photo-Fenton process. The Fenton reaction is seen to strongly correlate with
the post-TiO 2 photocatalysis reaction and thus is described in detail here. The mechanism for the Fenton reaction is shown in Eq. (13.13):
Fe aq H O
Fe aq OH HO
2
2 2
3
+
+
−
⋅
( )+
→
( )+
+
(13.13)
The Fe
2+
can be reverted back to Fe
3+
via different mechanisms:
Fe aq H O
Fe aq HO H
3
2 2
2
2
+
+
⋅
+
( )+
→
( )+
+
(13.14)
Fe aq HO
Fe aq O H
3
2
2
2
+
⋅
+
+
( )+
→
( )+ +
(13.15)
When a light source is present, the rate of photo-Fenton was reported to be
positively enhanced compared to the dark condition. This is mainly due to the
regeneration of Fe
2+
(aq) from the photochemical effect of light and the concurrent generation of the OH
•
radicals in the system. Such a reversion cycle of
Fe
2+
(aq) → Fe
3+
(aq) → Fe
2+
(aq) continuously generates HO
•
, provided that the
concentration of H 2 O 2 in the system is substantial. The regeneration of the Fe
2+
(aq) from Fe
3+
(aq) is the rate-limiting step in the catalytic iron cycle, if small
amount of iron is present. This photoassisted reaction is termed as photo-Fenton
reaction, where such reactions could be activated by irradiation wavelengths of
up to 600 nm. It was known that this reaction is better functional under longer
wavelengths as they are able to overcome the inner filter effects by photolyzing
Recent Developments in Photocatalytic Water Treatment Technology
Homogeneous Photo-Fenton Reaction
The Fenton reaction is a process that does not involve any light irradiation as compared with the heterogeneous TiO 2 photocatalysis reaction, whereas the photoFenton does react up to a light wavelength of 600 nm. It was first recognized in the
1960s and remains one of the most applied AOPs for its ability to degrade high
loading of organic compounds in highly saline conditions [17, 202, 243]. Numerous
studies on the photo-Fenton degradation of water pollutants such as chlorophenol,
pesticides, and phenolic or aromatic compounds with organic loading of up to
25 g/L have been investigated [88, 107, 108, 134, 259]. A number of literatures have
provided a comprehensive review of the basic understanding and clarity of the principles underlying the Fenton reaction [111, 243, 262].
In the absence of a light source, hydrogen peroxide (H 2 O 2 ) will be decomposed
by Fe
2+
ions that are present in the aqueous phase, resulting in the formation of
hydroxyl radicals. The photo-Fenton reaction is expedited when light source is present, causing rapid H 2 O 2 decomposition by ferrous or ferric ions and resulting in the
formation of radicals. All these soluble iron hydroxy or iron complexes can absorb
not only UV radiation but also visible light. However, the actual oxidizing species
responsible for the photo-Fenton reaction is still under discussion [42, 73]. These
Fenton and photo-Fenton reactions could occur simultaneously with TiO 2 photocatalysis during UV-Vis irradiation period, post-TiO 2 photocatalysis period, or standalone photo-Fenton process. The Fenton reaction is seen to strongly correlate with
the post-TiO 2 photocatalysis reaction and thus is described in detail here. The mechanism for the Fenton reaction is shown in Eq. (13.13):
Fe aq H O
Fe aq OH HO
2
2 2
3
+
+
−
⋅
( )+
→
( )+
+
(13.13)
The Fe
2+
can be reverted back to Fe
3+
via different mechanisms:
Fe aq H O
Fe aq HO H
3
2 2
2
2
+
+
⋅
+
( )+
→
( )+
+
(13.14)
Fe aq HO
Fe aq O H
3
2
2
2
+
⋅
+
+
( )+
→
( )+ +
(13.15)
When a light source is present, the rate of photo-Fenton was reported to be
positively enhanced compared to the dark condition. This is mainly due to the
regeneration of Fe
2+
(aq) from the photochemical effect of light and the concurrent generation of the OH
•
radicals in the system. Such a reversion cycle of
Fe
2+
(aq) → Fe
3+
(aq) → Fe
2+
(aq) continuously generates HO
•
, provided that the
concentration of H 2 O 2 in the system is substantial. The regeneration of the Fe
2+
(aq) from Fe
3+
(aq) is the rate-limiting step in the catalytic iron cycle, if small
amount of iron is present. This photoassisted reaction is termed as photo-Fenton
reaction, where such reactions could be activated by irradiation wavelengths of
up to 600 nm. It was known that this reaction is better functional under longer
wavelengths as they are able to overcome the inner filter effects by photolyzing
Recent Developments in Photocatalytic Water Treatment Technology